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Speciation and Recessive Mutations for BMC Biology Essay: On the close relationship between speciation, inbreeding and recessive mutations. Etienne Joly, [email protected], Toulouse, September 2010 All the ideas developed in this essay are relatively simple, and most of them are related to many Foreword previously published works. So much work, however, This past year, 2009, was the Darwin year, celebrating has already been published on evolution and speciation the 200th anniversary of Charles Darwin's birth, and 150 that an autodidactic newcomer such as myself could years since the publication of his fabulous milestone not hope to read, let alone understand and remember all book, ‘The Origin of Species’ (to which I will the primary papers published previously on evolution subsequently refer to as ‘The Origin’). At the start of and speciation. If I have failed to acknowledge 2009, I was inhabited by a nagging ethical concern : how previous works developing ideas related to those put would humans deal with a situation where a group of forward here, the reader can be assured that this was individuals found themselves fertile among one another, not done maliciously but simply as a result of my but with limited fertility with the rest of the human race ? relative naivety on the subject. I do, however, hold the In other words, could speciation occur within the human firm conviction that, if some of the ideas developed in race ? This concern sprouted from the idea that this essay prove to be correct and relatively novel, it chromosomal rearrangements seemed to me like a very was only possible because of this naivety. probable initial step of a speciation process, since systematic survey of the human populations have actually Because, as a rule, I have adopted the principle of shown that such rearrangements are relatively frequent ( never citing a paper that I have not read, numerous frequency of the order of 1/1000, [1] ). Furthermore, times during the writing of this essay, I have found given the amazing success of the human race, having myself unable to decide what particular paper to cite as resulted in the huge numbers of human beings currently the appropriate original source of a particular concept living on our planet, and given the amazing propensity of or observation. Although I have tried to read as many nature to generate new species, I felt that the chances primary papers as I could rather than reviews, I found must be quite high that speciation could occur within the that I simply could not read everything. In addition, human population. most books, and also many papers were not available Most scientist concerned with evolution and to me in our institute's library or freely online (As speciation would probably not share those concerns another rule, I refuse to pay for online access, because I because the commonly held view is that speciation is firmly believe that all primary research papers should most often allopatric, i.e. it occurs when populations of be freely available to all). In such situations when I had individuals evolve separately from one another for a not managed to read the primary papers (for whatever sufficiently long time that they would no longer breed reason), I have very often chosen to cite the very efficiently with one another when they are reunited, comprehensive et quite recent reference book and the mobility of modern humans would be very "Speciation" by Coyne and Orr (2004), and to refer to effective at countering this type of phenomenon. it as ‘C&O’, with the indication of the appropriate Contrarily to these views, I believe that, if chapter or page number. reproductive barriers arise, they are basically Nature Precedings : hdl:10101/npre.2010.5003.2 Posted 20 Oct 2010 equivalent to other characters, and must therefore have The ideas presented here are relevant to many, if not been selected for. In other words, there must be most, subjects related to speciation and evolution. conditions where it becomes advantageous for groups Another difficulty I have encountered in the writing of of individuals to stop breeding with the ancestral this essay is that of keeping the discussion focalised on population. The main conclusion that I reach by the the main subject of this essay, i.e. that new species do reflections developed in this essay is that the process of not arise by splitting in separate branches, but by speciation is primarily driven by advantageous “budding” away from the ancestral population as a recessive mutations, and that those will, in turn result of inbreeding among a limited number of promote reproductive barriers, favouring inbreeding. individuals. As a solution, for discussing several issues This past year has seen the publication of a plethora that seemed either too technical or rather peripheral to of review articles on the subject of evolution and this main subject, I have chosen to treat those as speciation, which have allowed me to start catching up separate short pieces which are provided either as on these vast subjects, and to mature my reflections on footnotes, or as addenda at the end of this essay. the mechanisms involved in speciation. The reading of these reviews had also allowed me to confirm that More complex subjects, such as evolvability, the those ideas I have developed are in disagreement with origins of life and of sexual reproduction, group level the generally held views, i.e. that allopatric speciation selection and altruism would, however, each have is the most common and probable route for the warranted whole essays, and it was beyond my appearance of new species. capacities to cover all those issues simultaneously. I have thus had to put back dealing with those other issues to later dates. 28/09/10 1 Abstract: One of the important reasons for this failure was related to an issue to which he alluded to repeatedly in Whilst the principle of adaptive evolution is his book, which is that species are basically impossible unanimously recognised as being caused by the process to define. The main problem, which he acknowledged of natural selection favouring the survival and/or himself, and stays whole today, lies in the fuzzy limit reproduction of individuals having acquired new between species and varieties: “From these remarks it advantageous traits, a consensus has proven much will be seen that I look at the term species, as one harder to find regarding the actual origin of species. arbitrarily given for the sake of convenience to a set of Indeed, since speciation corresponds to the individuals closely resembling each other, and that it establishment of reproductive barriers, it is difficult to does not essentially differ from the term variety, which see how it could bring a selective advantage because it is given to less distinct and more fluctuating forms. The amounts to a restriction in the opportunities to breed term variety, again, in comparison with mere with as many and/or as diverse partners as possible. In individual differences, is also applied arbitrarily, and this regard, Darwin himself did not believe that for mere convenience sake.” (The Origin, p. 52 mid Ch reproductive barriers could be selected for, and today II). most evolutionary biologists still believe that One of the most important concepts that derives from speciation can only occur through a process of the work of Darwin is that the process of life is one of separation allowing two populations to diverge constant evolution, which explains why so few of the sufficiently to become infertile with one another. I do, life forms that occupied the earth 20 millions ago are however, take the view that, if so much speciation has still around today. The somewhat uncomfortable but occurred, and still occurs around us, it cannot be a inescapable conclusion from this is that the existence consequence of passive drift but must result from a of every single one of the millions of species that selection process, whereby it is advantageous for surround us, including ours, must also be transitory, groups of individuals to reproduce preferentially with and this probably contributes to the difficulty that one another and reduce their breeding with the rest of many humans have in accepting the theory of the population. evolution, in addition to the fact that it also brings In this essay, I propose a model whereby new species serious questions as to the existence of an almighty arise by “budding” from an ancestral stock, via a God. The processes of evolution and speciation are, process of inbreeding among small numbers of however, very slow ones, and the 5000 years of human individuals, driven by the occurrence of advantageous history1 do not amount to even a tick on the clock of recessive mutations. Since the phenotypes associated to evolutionary times, and to our human eyes, the stability such mutations can only be retained in the context of of the world thus appears as if it should stay the same inbreeding, it is the pressure of the ancestral stock for ever, and so with the species that occupy it. The which will promote additional reproductive barriers, fact that species are not stable entities, but in constant and ultimately result in complete separation of a new evolution is another factor that adds to the difficulty of species. I thus contend that the phenomenon of defining them. speciation would be driven by mutations resulting in Initially, species were recognised and defined by the advantageous loss of certain functions, whilst naturalists and palaeontologists mostly in relation to adaptive evolution would correspond to gains of their anatomical features, and it is on the basis of these Nature Precedings : hdl:10101/npre.2010.5003.2 Posted 20 Oct 2010 function that would, most of the time be dominant.
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